Tangential cutting inserts, are oriented in an insert holder in such a manner that during a cutting operation on a workpiece the cutting forces are directed along a major (thicker) dimension of the cutting insert. An advantage of such an arrangement is that the cutting insert can withstand greater cutting forces than when oriented in such a manner that the cutting forces are directed along a minor (thinner) dimension of the cutting insert.
There is disclosed in EP 0 769 341 a face milling cutter employing a double-sided indexable tangential cutting insert having a prismoidal shape with two opposed generally rectangular rake surfaces connected by side surfaces. The cutting insert has a basic “negative” geometry and therefore in order to provide the necessary clearance between the cutting insert and the workpiece, when mounted in a face-mill, the cutting insert is oriented with a negative axial rake angle.
A double-sided indexable tangential cutting insert for a boring tool head is disclosed in U.S. Pat. No. 5,333,972. The insert is provided at each end with a protruding flat island. Each long cutting edge is inclined at an angle of 3° relative to the protruding flat island, defining an “insert axial rake angle”. Rearward of each cutting edge is a descending land surface that merges with an increasing incident angle surface to form a chip breaker groove. Each increasing incident angle surface extends from its associated descending land surface to an adjacent island, at either the top or the bottom of the cutting insert. The cutting insert is left or right handed. It is manufactured to be right-handed and, when flipped around, is left-handed. It will be appreciated that the magnitude of the insert axial rake angle is limited for practical reasons. Any increase in the insert axial rake angle will result in an increase in the “vertical” extent of the increasing incident angle surface (see FIG. 3 of U.S. Pat. No. 5,333,972) that will have an adverse effect on chip development and evacuation.
There is disclosed in U.S. Pat. No. 6,238,146 a tangential cutting insert having front and rear surfaces of a generally concave shape. Opposite upper and lower surfaces and a pair of opposite side surfaces extend between the front and rear surfaces. There are four main cutting edges that are concave in a side view of the cutting insert. Each main cutting edge extends between two corner cutting edges. Each major cutting edge merges with an associated auxiliary cutting edge via a corner cutting edge. All the cutting edges are associated with the front and rear surfaces which have rake surfaces formed in them along which chips cut from a workpiece flow during a cutting operation. The upper, lower surfaces and side surfaces do not have rake surfaces formed in them and there are no cutting edges associated with the upper, lower surfaces and side surfaces.
Although the cutting inserts shown in the various embodiments in U.S. Pat. No. 6,238,146 have eight cutting corners, it is only possible to use all eight cutting corners if only half the length of the main cutting edges is used. Cutting edges become worn during use. Moreover, in U.S. Pat. No. 6,238,146 each main cutting edge has associated with it two cutting corners, one at each extremity. Hence, if more than half the length of the main cutting edges is used, say in milling a shoulder in a workpiece, then when using a second cutting corner of a main cutting edge, for which the first corner has already been used, only a small portion of the main cutting edge adjacent the second cutting corner will be new, the rest of the main cutting edge will be used and worn.
It is an object of the present invention to provide an indexable tangential cutting insert having eight main cutting edges, each main cutting edge having a single cutting corner associated with it.
It is a further object of the present invention to provide an indexable tangential cutting insert capable of milling a square shoulder in a workpiece.